This invention relates to a speed detection apparatus and method for detecting speed information using position pulses obtained from a position detector. More particularly, the invention relates to a speed detection apparatus and method wherein speed can be detected with great accuracy over a wide range.
Controlling the speed of an A.C. or D.C. motor requires that the actual motor speed be detected and compared with a commanded speed. FIG. 1 is a block diagram of a common servo control system. A servo motor 1 has a rotary encoder (position detector) 2 connected directly to its rotary shaft for producing position pulses PP, each of which is generated whenever the motor rotates by a predetermined amount. The pulses PP are applied to an arithmetic circuit 3, which proceeds to compute the difference between the number of pulses PP generated and a number of position command pulses PCMD obtained from an external unit, the computed difference being stored in an error register 4. The digital data within the error register 4 is converted into an analog voltage by a digital-to-analog converter (D/A) 5, and the resulting voltage is applied as a speed command voltage to a speed control circuit 7. Meanwhile, using the position pulses PP from the rotary encoder 2, a speed detecting circuit 6 detects the actual speed of the motor 1 and provides the speed control circuit 7 with a voltage equivalent thereto. The speed control circuit 7 controls the speed of the servomotor 1 based on the difference between the speed command voltage and the voltage signal corresponding to the actual speed. In a servo control operation for performing control based on speed feedback in the above-described manner, the accuracy of speed feedback control depends upon the speed detecting precision of the speed detecting circuit 6. The accuracy of speed detection is therefore extremely important in servo control.
A speed detection apparatus heretofore available in the art has a position detector for generating two signals displaced in phase from each other by .pi./2 and having a frequency f proportional to the rotational speed of the motor. The two-phase signals are converted into signals of a frequency 4 f by means of a quadrupling circuit. Finally, a frequency-to-voltage converter generates a voltage proportional to the frequency 4 f, that is, a voltage (actual speed voltage) proportional to the rotational speed of the motor.
A problem encountered in the foregoing prior-art arrangement is that, when the motor speed drops to a low value, the magnitude of the voltage signal produced by the frequency-to-voltage converter no longer remains proportional to the rotational speed of the motor but instead exhibits a sudden decline which diminishes the accuracy of measurement. Accordingly, rather than relying upon a frequency-to-voltage conversion, methods of detecting speed by digital processing using a microcomputer have been proposed to increase accuracy at a reasonable cost in view of LSI techniques.
One such method performs detection by counting position pulses applied within a fixed period of time and then treating a value proportional to the counted value as speed information. When the fixed period is shortened, however, the number of pulses applied within said period diminishes so that there is a decline in the accuracy of speed detection. This makes it necessary to lengthen the period. However, a longer period also lengthens detection time, thereby detracting from the accuracy and response of the speed detection system.
A second detection method performs detection by monitoring the interval (i.e., period) between position pulses and then treating the reciprocal of the interval as speed information. However, the period of the position pulses shortens at high-speed rotation so that it becomes difficult to measure the period accurately under such conditions.